1. Technical Field
The present disclosure relates to radio frequency (RF) circuit components, and more particularly, to a miniature RF directional coupler.
2. Related Art
Directional couplers are passive devices utilized to couple a part of the transmission power on one signal path to another signal path by a predefined amount. Conventionally, this is achieved by placing the two signal paths in close physical proximity to each other, such that the energy passing through one is passed to the other. This property is useful for a number of different applications, including power monitoring and control, testing and measurements, and so forth.
The directional coupler is a four-port device including an input port (P1), an output port (P2), a coupled port (P3), and an isolated or ballasting port (P4). The input power of RF signal supplied to P1 is coupled to P3 according to a coupling factor that defines the fraction of the input power that is passed to P3. The remainder of the power on P1 is delivered to P2, and in an ideal case, no power is delivered to P4. The degree to which the forward and backward waves are isolated is the directivity of the coupler, and again, in an ideal case, would be infinite. Directivity may also be defined as the difference between S31 (coupling coefficient) and S32 (reverse isolation). In an actual implementation, however, some level of the signal is passed to both to P3 and P4, though the addition of a ballasting resistor to P4 may be able to dissipate some of the power.
The type of transmission lines utilized in such conventional RF directional couplers includes coaxial lines, strip lines, and micro strip lines. The geometric dimensions are proportional to the wavelength of transmitted signal for a given coupling coefficient. Directional couplers utilizing lumped element components are known in the art, but such devices are also dimensionally large. These devices are implemented with ceramic substrates and thin-film printed metal traces, and have footprints of 2×1.6 mm and 1.6×0.8 mm and above, which is much larger than semiconductor die implementations. Notwithstanding the relatively large physical coupling area of the transmission lines, such directional couplers only have a directivity of around 10 dB. The resultant power control accuracy is approximately +/−0.45 dB. Such performance is unsuitable for many applications including mobile communications, where high voltage standing wave ratios (VSWR) at the antenna are possible.
Instead of lumped element circuits, directional couplers may be based on integrated passive devices (IPD) technology and implemented on wafer level chip scale packaging (WL-CSP). Due to the footprint restrictions, implementation of directional couplers on semiconductor dies is generally limited to microwave and millimeter wave operating frequencies. These types of directional couplers utilize two coupled inductors. Although suitable for on-die implementations, such couplers exhibit low levels of directivity due to the small geometric dimensions. With a mismatch on the output port (P2), the reflected signal may leak to the coupled port (P3) and mix with the originally coupled signal, thereby resulting in a high level of uncertainly in measurements of transferred power to the output port P2. Even with higher coupling coefficients possible with increasing the number of turns in inter-wound micro strip line coupled inductors, directivity remains low.
An improvement over the basic coupled inductor architecture is disclosed in U.S. Pat. No. 7,446,626. In addition to the coupled inductors, there is a compensation capacitor and a compensation resistor that are understood to provide a high level of directivity (around 60 db) notwithstanding the small geometry. With the use of low inductance values, low insertion loss resulted. However, there are several deficiencies with such earlier directional couplers. The lumped element capacitors utilized therein are only capable of sustaining a limited voltage level. In typical metal-insulator-metal (MIM) capacitors, the breakdown voltage ranges from 5V to 30V, depending on the particular semiconductor technology utilized. Conventional techniques for increasing capacitive density involve reducing the thickness of the dielectric between the metal plates to several hundred angstroms, and though the footprint is reduced, so is the breakdown voltage. The use of the aforementioned compensation resistor for achieving high directivity across a wide frequency range is also problematic in that a more expensive semiconductor process must be utilized. It is possible in some instances to exclude the compensation resistor, but this results in reduced directivity.
Further improvements to directional couplers are disclosed in U.S. patent application Ser. No. 13/333,706 entitled ON-DIE RADIO FREQUENCY DIRECTIONAL COUPLER filed on Dec. 21, 2011 and published as U.S. Pat. App. Pub. No. US 2012/0161898 on Jun. 28, 2012, the entirety of which is incorporated herein by reference. This disclosure utilizes two coupled inductors and two or three compensation capacitors. Utilization of compensation capacitors allows for high voltage operation of these couplers. This allowed for a relatively small size with reasonable performance. However, for cellular (WCDMA and the like) designs, it would be desirable to have a coupling coefficient of approximately 20 dB, which would result in a fairly large directional coupler and higher associated insertion loss if this design were followed.
Therefore, there is a need in the art for an improved RF directional coupler capable of being used in cellular applications with a high level of directivity and a miniaturized size in comparison to the prior art for reduced insertion loss.